Although cyanides themselves are useful industrial materials, as a component of waste water certain forms of cyanide are undesirable, having been found to be offensive to the aquatic environment. Examples of the undesirable forms of cyanides are HCN and CN.sup.-. As an environmentally protective measure, the Environmental Protection Agency (EPA) has placed strict limits on the allowable cyanide levels of industrial waste water effluent streams. Industrial sources of undesirable forms of cyanide are waste waters from the coal, the natural gas and the electroplating industries. Refinery waste waters, mainly from the FCC and coking processes, also require treatment to remove cyanides.
Various approaches to removing cyanides have been described. A process which utilizes hydrogen peroxide for the destruction of cyanide in waste water has been found to meet the cyanide effluent limits. However, the process requires continuous addition of hydrogen peroxide which is costly. Additionally, storage and handling of the hydrogen peroxide requires extensive procedures and equipment. Hydrogen peroxide decomposes readily and special care must be taken to avoid contamination of the storage vessel which could catalytically accelerate decomposition. Furthermore, because decomposition releases oxygen, which is explosive under pressure, the vessel must be carefully monitored to keep the oxygen level low.
Another water treatment method to remove cyanide is chlorination which converts cyanide to the essentially nontoxic cyanate. Chlorination processes typically use chlorine gas or hypochlorite. The disadvantage of chlorination is the cost associated with continuously supplying the source of chlorine to the waste stream which competes with other processes that utilize chlorine. Moreover, the chlorine requirements depend on the cyanide level in the water necessitating close monitoring to make adjustments in the chlorine concentration. Furthermore, the process, requiring refrigeration when recycling the HCN stripper bottoms back to the absorber for the most efficient waste volume reduction, increases the amount of energy and process equipment requirements.
One additional method for treating cyanides in waste water, particularly cyanides found in the effluent from the FCC and coking units, involves injecting sulfur-containing compounds such as polysulfides into the cyanide containing water. The polysulfides convert the cyanide to thiocyanate which can be stripped from the waste water. A disadvantage of this process, however, is the difficulty in meeting the low conversion requirements of the EPA specifications.
U. S. Pat. No. 3,650,949 teaches a method of removing cyanide from waste water by maintaining in the water high levels of cupric ion and oxygen and passing the water containing the cyanide, cupric ion and oxygen into a bed of activated carbon. The source of cupric ion is a copper salt which is water soluble. The copper ion must be continuously added to the process stream to maintain the level of copper necessary to remove the cyanide. The patent teaches that a bed of activated carbon can be pre-impregnated with cupric ion; however, the water solubility of the cupric ion imparts serious economic disadvantages to the process. The copper will leach out of the bed leaving a residue in the treated stream requiring another treatment step to remove the residual copper. Also, leaching out of the copper requires re-impregnating the carbon bed. The major disadvantage of having to re-impregnate the bed includes the nonproductive time spent replacing the copper and the chemicals and equipment required.
In developing water treatment processes particular concern is directed to processes which do not leave residues in the treated stream. Residues can cause additional disposal problems. Materials consumption and cost is also an important factor; thus, it is important to avoid processes which require replenishing the supply of costly catalyst and reagent.